Hot Tub Volume Calculator (Gallons from Shell Size, Minus Seats)

Volume is the shell's top-face area multiplied by the water depth, then converted to US gallons at 7.48052 gal/ft³ (NIST: 1,728 in³/ft³ ÷ 231 in³/gal = 7.480519..., rounded to six significant figures). Optional seat and footwell displacement is subtracted from the gross volume. Liters are derived from gallons at 3.785412 L/gal (NIST).

square:   L x W x depth x 7.48052
round:    pi x (diameter/2)^2 x depth x 7.48052
octagon:  2(sqrt(2) - 1) x width^2 x depth x 7.48052
          width = flat-to-flat (between parallel sides)

net gallons = gross gallons - seat displacement - footwell displacement
liters      = net gallons x 3.785412   (NIST: 1 US gal = 3.785412 L)
cubic feet  = net gallons / 7.48052

The octagon factor 2(√2 − 1) ≈ 0.8284 converts the flat-to-flat width of a regular octagon into its area. This is the standard geometry for eight-sided spa shells. Measure between two parallel flat sides of the octagon, not corner to corner.

Seat and footwell displacement is an operator estimate, not a computed constant, because seat geometry varies by make and model. Leave the fields blank for the gross shell volume. Compare your result with the manufacturer's listed capacity to confirm the displacement estimate.

This tool uses 7.48052 gal/ft³, a six-significant-figure rounding of the NIST exact conversion. The pool suite's pool volume calculator pins 7.48 (a two-decimal rounding of the same value). The gap is about 0.007%, about 0.03 gallons on a 500-gallon tub, far below what a tape measure can resolve.

Sources: derived from NIST Guide to the SI, Appendix B.9: 2.831685E−02 m³/ft³ ÷ 3.785412E−03 m³/gal = 7.48052. 1 US gal = 3.785412 L (directly from B.9).